Simple table normalisationI've written about it here and on SO. Think about a "lookup" table--one column of data and an id number. Now drop the id number, write foreign keys to reference the data, and you avoid a join altogether. With other tables, you avoid a join every time the data you need is in a referenced key. (I was surprised how often this happens.) The tradeoff is between join performance and disk I/O. Prototypes indicated that natural keys would perform better in the test suite (dozens of queries) for decades without improvements in hardware (like SSDs).